[X86] Only extract constant pool shuffle mask data with zero offsets

Description

[X86] Only extract constant pool shuffle mask data with zero offsets

D53306 exposes an issue where we sometimes use constant pool data from bigger vectors than the target shuffle mask. This should be safe to do, but we have to be certain that we're using the bottom most part of the vector as the shuffle mask decoders have no way to peek into subvectors with non-zero offsets.